The problems of extending helix-type TWT's to high frequencies at high average power levels involve limitations of thermal dissipation and the small size and cost of delay line fabrication.
Conventional PPM-focussed TWT's of the helix type for wide-band operation are believed to be impractical for use above 50 GHz because of the difficulty and cost in fabricating parts of very small dimension. Engineering materials available for those devices have practical limits to the thermal dissipation densities in removing Joule heating of the helix structure, normally support by dielectric rods, in order to minimize circuit dispersion and to maintain broad band characteristics. The upper limit to power levels of operation is observed to decrease inversely as the five-halves power of frequency.
A number of alternative interaction structures have been developed for increased frequency and power performance. However, those non-helix-based structures are intrinsically bandwidth limiting. For example, linear beam interaction structures employing coupled cavities, folded waveguides, fast wave guides, helical waveguides, and carp lines eliminate the need for a dielectric support structure for much improved thermal dissipation. But, these circuits have not demonstrated the bandwidth properties of the conventional helix circuit in practical devices.
To circumvent the preceding difficulties, the TWT of this invention has a coaxial helix structure which interacts with a coaxial periodic-permanent-magnet(PPM)-focussed hollow beam. Analysis indicates that substantial power (hundreds of watts) is available from the coaxial TWT (CTWT) at frequencies above 100 GHz at substantial bandwidth (40% or better).
In the CTWT device of this invention, the problem of thermal dissipation limitations is avoided by employing a coaxial multifilar (10 to 40 wire starts per turn), contrawound helix structure having a greatly increased radius of cross section; this system of helices may be photolithographically deposited upon thin-walled dielectric tubing, typically quartz, providing an efficient thermal contact for heat dissipation as well as part of a vacuum envelope for the hollow beam. RF power dissipation densities are reduced inversely as the median radius of the helix system.
The low dispersive properties of the coaxial helix structure are achieved by means of the heavy back wall loading afforded by the metallic conductive layer bonded to the exterior surfaces of the dielectric tubing. Frequency extension of the helix system is a result of a property of the multifilar helix whereby the space harmonic structure in the Brillouin zones are extended in phase space (.beta. in an .omega.-.beta. diagram) in direct proportion to the number of helix tapes ("wires") encountered per turn (wire starts per circumferential revolution). Expressed in terms of the helix pitch, p, and the axial propagation constant, .beta., the first Brillouin zone defined by .beta.p/2.lambda. is increased to a value N .beta.p/2.lambda. where N is the number of wires per turn of the multifilar helices. Typical values of N for a 35 GHz circuit having a median helix diameter of 6 mm would be on the order of 30 for a 10 kV circuit structure in order to maintain the predominance of the fundamental spatial harmonic and to avoid backward wave oscillation difficulties.
A feature of this invention is that the space charge potential within a radial cross-section of the beam is small, typically less than 10 volts, and is a result of having the beam confined between the inner and outer multifilar helical coils forming the coupled slow-wave circuits. The low space charge potential allows focussing of the beam by a PPM structure whose required magnetic field is substantially smaller than is required for a single helix TWT whose space charge potential within a radial cross-section of the beam is one or two orders of magnitude larger.